Asymmetric Hydrogenation of Ketones Catalyzed by Ru~(II)-bicp Complexes

摘要

Enantiomerically pure secondary alcohols are among the most valuable key intermediates for the manufacture of pharmaceuticals and advanced materials. The simplest and most powerful way to produce chiral alcohols is the asymmetric hydrogenation of ketones. The most general and efficient catalyst system for the enantioselective hydrogenation of a variety of simple ketones reported so far is Noyoris homochiral Xylbinap/daipen/RuII combination with iPrOH as the solvent and in the presence of tBuOK. The high degree of enantioselectivity is a result of the synergistic effects of the chiral diphosphane and diamine ligands. The Ru complexes of the parent phosphane ligand of the series, binap (2,2-bis(diphenylphosphanyl)-1,1'-binaphthyl), afford significantly lower selectivities in this reduction. Studies have been reported showing that Noyori's homogeneous hydrogenation follows a nonclassical mechanism, whereby a hydride on the Ru center and a proton of the NH_2 ligand are transferred simultaneously to the C=O function through a six-membered pericyclic transition state.